In recent years, a technique (“metamaterial” technique) for controlling propagation characteristics of electromagnetic waves by periodically arranging conductor patterns having a specific structure has been proposed. In particular, it has been proposed that a technique called “frequency selective surface” to control transmission characteristics and reflectance characteristics of incident electromagnetic waves by a configuration in which specific conductor patterns are two-dimensionally periodically arranged as unit cells be applied to a Radome (radar dome) for a reflector of an antenna or a radar device.
In an “antenna device” described in Patent Literature 1 (Japanese Unexamined Patent Application Publication No. 2011-217269), a configuration in which a frequency selective surface on which ring-like conductor patterns are periodically arranged is provided on a back surface of an antenna is disclosed. The frequency selective surface disclosed in Patent Literature 1 can reflect electromagnetic waves only at antenna operating frequencies and transmit electromagnetic waves in the other frequency bands, thereby enabling a reduction in a radar cross section in the frequency bands other than the antenna operating frequencies.
Further, in an “antenna for reducing unwanted radiation” described in Patent Literature 2 (Japanese Unexamined Patent Application Publication No. 2003-60430), a configuration in which a frequency selective surface on which metallic pieces are periodically arranged is provided on a front surface of an array antenna is disclosed. The frequency selective surface disclosed in Patent Literature 2 can transmit electromagnetic waves only at antenna operating frequencies and reflect electromagnetic waves in the other frequency bands, thereby enabling a reduction in radiation at unwanted frequencies from the array antenna.